There are now several different common types of optical media for storing information. The information stored on such media may include music signals, computer data, text, images, video and movies, etc. Among the common types of optical media are media based upon phase change (PC) materials.
In the context of optical recording media, PC materials are materials which can be made to exist stably at room-temperature, and within a useful operating and storage range about room temperature, in at least two distinguishable forms. For example, some PC materials can be made to exist alternatively in a stable amorphous state and in a stable crystalline state. These stable states can be distinguished using optical recording and reading apparatus, for example on the basis of differing reflectivities.
PC materials can be made to change state from one stable state to another stable state by heating them. Some such materials irreversibly change state when heated, while others change from one state to another when heated a first amount and change back when heated a second, different, amount. The heat is usually applied by a laser light source, which can also be used, at a lower output power level to detect in which state the material was most recently left. Since heat is used to change the state of the material for the purpose of recording information therein, the layer is sensitive to extremes of heat, which can cause information to be altered or lost.
In optical recording systems, such as disk drives, the recording medium, i.e. disk, is made to spin at a high speed, e.g. 3,600 RPM, while an optical system directs a laser beam onto the PC layer for reading and writing information therein. Impact or contact with the surface of such a rapidly spinning disk can generate sufficient heat to cause alteration or loss of the information recorded in the vicinity of the impact or contact. Therefore, common PC media locate the layer of PC material beneath and protected by the substrate which provides substantial mechanical support for the media. Locating the PC material in a position so that the laser must pass through the substrate affects the optical system used to focus the laser.
Disk drives for use with common PC media employ an optical system which locates the closest element to the spinning disk at a substantial, mechanically fixed distance from the disk. The mechanism which holds the closest element at the proper mechanically fixed distance from the disk is known as a non-flying head, to distinguish it from a mechanism used in connection with many magnetic and magneto-optical media known as a flying head.
A flying head maintains a substantially closer spacing from the spinning disk by using aerodynamic principals to "fly" over the surface of the disk. However, there is a substantial risk that a flying head will inadvertently contact the surface of the disk, causing information to be altered or lost. Moreover, a flying head compatible with focussing the laser through the substrate, called substrate incident operation, would not be capable of carrying a sufficiently large lens and also following perturbations of the disk surface called vertical runout. Hence, flying heads have not been used in connection with PC media. One system, disclosed by Ukita et al. in "Supersmall flying optical head for phase change recording media," Applied Optics, Vol. 28, No. 20, Oct. 15, 1989, employs a flying head which does not focus the laser through the substrate, called air incident operation. However, this head is made to fly at a height of a few micrometers, stated to be ten times greater than the flying height of a magnetic head. The medium used, however, is substantially unprotected from impacts by the head.